Is it even stated outright that Silvadi is the evolution of Type: Null, or just that Type: Null turns into Silvadi after some condition is fulfilled?
If the latter is the case, you could argue that Type: Null is not a Pokémon per se. Just a "pseudo-Pokémon", or a form of Silvadi, akin to Pokémon XD's Shadow Lugia, named XD001.
If I may hypothesize: Type: Null will only be known as Type: Null before you obtain it. It will fight under Gladion's command, but with reduced stats compared to the "real thing", and the Rotom dex will not recognize it (hence the name). Then later, it will break its seal, become free from the Aether Foundation or Team Skull or whatever, show its true colours as Silvadi, and only then will you be able to interact with it like a regular Pokémon.
Other comparisons would be the cocoon forms of Xerneas and Yveltal, the Marowak Ghost, or Mewtwo in its power armour from the first movie. Technically a creature in the franchise, it may even battle you at some point, but unobtainable in those forms. And when you get them later, there are no signs of their pre-transformation form to be found.